Associations between ambient air pollutants and bone turnover markers in children were explored with generalized additive models (GAM, MGCV package in R (Wood, 2004)), as non-linear associations were observed between some continuous confounders (BMI, age and physical activity) and the bone turnover marker concentrations. Air pollutant concentrations and the distance to a major road were included as linear terms, after testing their linearity with the bone turnover markers using GAM plots. The distance to a major road was also categorized in to ≤350 m versus >350 m.